Impact hammers are widely used in mining, digging, and demolition work. Their function is to apply high unit area impact loads repetitively to a surface to fragment it or to divide it. The common jackhammer is an example of a pneumatically-powered device driven by compressed air, which delivers sharp impact blows at the tip of a tool such as a pick or a spade.
While the jackhammer remains in widespread use, its application has gradually been reduced to relatively portable tools handled by a muscular individual. The reaction to these blows is exerted by the mass of the tool, and by the operator. This is an obvious limitation on the utility of this type of tool.
Accordingly, carriage-mounted pneumatic impact tools came into vogue, but it soon became apparent that while they could accommodate hammers which could deliver heavier blows, the hammers themselves became a limiting feature because of the inherent limitations of directly using a compressed gas for power. The volume of flow, the energy losses incurred in the compression-expansion cycle, and the inherent inefficiencies involved in the cycling of the gas through the hammer, among other complications, exerted an undesirable limit on the energy of the impacts that could be delivered, regardless of how suitably the hammer was mounted.
In response to these limitations, a liquid-powered class of impact hammers has developed during recent decades. Because the pressurized liquid used for powering the device is substantially non-compressible, many of the most troublesome problems of the pneumatic devices are avoided. The hoses, fittings and passages are sized to accommodate the liquid volume, and there are no significant losses caused by expansion, because there is no substantial expansion of the motive fluid itself.
The general theory of liquid-powered devices is to utilize a gas cell that is compressed by a pressurized liquid. The cell and the liquid which pressurizes it are held captive by a quickopening poppet valve. When the valve is opened, the pressurized liquid driven by the expanding gas cell is applied to a driven face of a hammer head. This is a very abrupt, high energy release situation. The driving pressure may be on the order of 2,000 psi or greater, and the effective area of the driven face may be on the order of at least 5 square inches to as much as 1,258 square inches.
In turn, the hammer head strikes a tool whose point or blade is usually at least several times smaller at the point of impact. The advantages of such an arrangement are obvious, and are reflected in the following exemplary United States patents:
______________________________________ U.S. Pat. No. Issue Date ______________________________________ 3,263,575 August 2, 1966 3,363,512 January 16, 1968 3,363,513 January 16, 1968 4,111,269 September 5, 1978 ______________________________________
Impact hammers of this general class are widely used, and in fact deliver blows of much greater impulse than pneumatically powered tools, even carriage mounted pneumatically powered tools.
In the continuing course of development of liquid powered impact hammers, problems have continually arisen which are not encountered in gas powered tools. The literature contains mention of many of them. Cavitation is one, liquid hammer effects are another. Most of these have been solved by one means or another, but there still remain the stubborn problems of reducing the flow of pressurized liquid to a sensible minimum, and of appropriately valving the flow of the liquid such that the fluid does not impede the loading or discharge of the tool, and so the tool does not destroy itself or have a degraded performance as the consequence of abrupt blows between the elements of the tool itself.
These problems have not yet previously been fully corrected. It is an object of this invention to provide in an impact hammer a flow and valving system for loading and discharging an impact tool which, while forgiving of external forces and effects still enables the tool reliably to be operated in a wide array of operating conditions on a near-minimum volume of liquid, with only minimal, if any, impediment to the loading and discharge of the tool, and without damaging internal blows between the elements of the impact hammer itself. It is intended that any sharp blow be only between the head of the hammer and the impact tool, and that this be exerted only over a very short stroke length.
As a further advantage, the above objectives are attained in an impact hammer which has a minimal number of parts, all of which are constructed with inherently stable shapes and substantial sections so as to resist the very strong and abrupt forces which are involved in the operation of this device.